1. Field of the Invention
The present invention relates to a method of manufacturing a high-quality thin metal strip by injecting a molten metal onto a cooling roll in an oxidized atmosphere gas. The presence of the oxidized atmosphere gas stabilizes the molten metal. Subsequent cooling and solidifying of the stabilized molten metal results in a thin metal strip having excellent surface properties.
2. Description of the Related Art
Conventionally, a thin metal strip is manufactured directly from a molten metal by causing the molten metal to flow from a nozzle onto the peripheral surface of a cooling roll which is rotating at a high speed. The molten metal is then cooled and solidified on the peripheral surface of the roll. This manufacturing method is classified as either a single roll method or a twin roll method.
The single roll method is suitable for the manufacture of a wide thin metal strip.
In the single roll method, a molten metal is injected from the nozzle toward the roll which is being rotated at a high speed and the molten metal forms a thin deposit on the surface of the roll. As the molten metal is cooled and solidified it becomes amorphous. This amorphous metal is then continuously peeled off the roll surface by the centrifugal force generated by the rotation of the roll to form the thin metal strip. The thin strip has a thickness of 50 .mu.m or less and is manufactured at a speed of 20 m/sec or more.
In the conventional casting, the atmosphere remains substantially at rest with respect to the molten metal and deterioration of the surface properties due to the engulfing of air by the metal does not occur. If deterioration of the surface properties does occur, however, the surface properties can be improved by descaling or rolling which is performed in a subsequent process.
The conventional casting techniques for improving the surface properties of metals cannot be used on thin metal strips manufactured by these methods. Descaling or rolling cannot be performed in the manufacture of a thin metal strip by the single roll method because manufacturing speed is very high and the thickness of the metal strip is very thin.
Japanese Patent Laid-Open No. 58-141837 provides a gas interrupting member having a passageway for a high-speed gas stream therein to interrupt the high-speed gas flow which is generated near the surface of the roll by the rotation of the cooling roll and which adversely affects the cooling roll, nozzle and molten metal puddle. The gas interrupting member is an inverted U-shaped thin tube. The opening of the passageway which is formed remote from the roll is an inlet, and the opening thereof located close to the roll is an outlet. The cross-sectional area of the interrupting member gradually reduces toward the outlet.
However, in the method disclosed in Japanese Patent Laid-Open No. 58-141837, although only a certain amount of air flow near the surface of the roll can be interrupted, a high-speed gas flow near the roll surface cannot be interrupted. The present inventors discovered that there was no change in the surface property between the thin metal strip manufactured by this method and the conventional thin metal strip.
Japanese Patent Laid-Open No. 54-50430 discloses the method of improving the surface property of a thin metal strip by manufacturing the same while absorbing the gas which adversely affects the surface of the thin strip near the roll, nozzle and molten metal puddle. The present inventors actually manufactured a thin metal strip by this method and found that turbulence of the gas near the roll and nozzle could be eliminated.
However, the turbulence of the high-speed gas flow which is associated with the improvement in the surface properties of the thin strip occurs at a position which is separated from the surface of the roll by 100 .mu.m or less, and the high-speed gas flow at such a site could not be eliminated. The manufactured thin metal strip had the surface property which was substantially the same as that of the conventional thin metal strip.
The methods of improving the surface properties of a thin metal strip by interrupting the flow of the gas near the surface of the roll described above cannot completely solve the problem involving the surface properties of the thin strip.
Japanese Patent Laid-Open No. 62-166058 discloses a new method of improving the surface properties of a thin metal strip by using a cooling roll mainly made of iron. This method is directed to eliminating the engulfing of gas into the interface between the cooling roll and the molten metal puddle formed on the cooling roll by the gas flow generated near the surface due to an increased wettability between the roll and the molten metal which is obtained when the thin metal strip mainly made of iron is manufactured.
The present inventors actually manufactured the thin metal strip by this method and found that some types of molten metal attached to the cooling roll made of iron-based metal. In such a case, the property of the manufactured thin strip was not excellent. The present inventors also found that cooling was not conducted completely due to a small coefficient of thermal conductivity when an amorphous thin metal strip was manufactured. A crystallized thin strip was obtained in place of the amorphous one. Also, the thus-obtained thin strip did not have a ribbon-like shape but a sea weed-like shape and was stretched at the two edges thereof. Although the obtained thin strip had better surface properties than the conventional thin strip, the properties of the entire thin strip was worse than the conventional one. This method cannot be applied to the manufacture of an amorphous thin metal strip because of occurrence of crystallization.
Hence, the following problems of the conventional techniques must be solved to improve the surface property of the thin metal strip.
(1) The most important issue is to prevent deterioration in the surface properties of the thin strip caused by a turbulence of the puddle produced on the surface of the cooling roll. The turbulence is due to the high-speed gas flow generated near the surface of the cooling roll by the rotation thereof.
(2) The second issue is to satisfy the general properties and magnetic characteristics of the thin metal strip.